As an example of a conventional high-frequency heating apparatus, a microwave oven will be described below. An example of the configuration of a conventional microwave oven is shown in FIG. 6. A mechanical time-limiting device 1″ has the following components assembled into a unit: a heating duration setter 1a, a bell 1b, a mechanical switch functioning as a time-limiting switch 1e, and a mechanical switch functioning as a time-limiting switch 1f. One end of commercial alternating-current power source 2 is connected via the time-limiting switch 1e to one end of the time-limiting switch 1f. The other end of the time-limiting switch 1f is connected via a surge circuit 10 to one end of a primary coil of a high-voltage transformer 5. On the other hand, the other end of the commercial alternating-current power source 2 is connected directly to the other end of the primary coil of the high-voltage transformer 5.
The surge circuit 10 is composed of a surge input monitoring circuit 11, a switch 12, and a resistor R2. One end of the surge input monitoring circuit 11, one end of the switch 12, and one end of the resistor R2 are connected to the time-limiting switch 1f. The other end of the switch 12 and the other end of the resistor R2 are connected to the primary coil of the high-voltage transformer 5. The other end of the surge input monitoring circuit 11 is connected to the node between the other end of the commercial alternating-current power source 2 and the other end of the primary coil of the high-voltage transformer 5.
Moreover, one end of electric circuitry 3 (hereinafter referred to as the high-frequency heating oscillator cooling device and other components 3) including components—such as an oven lamp for illuminating the interior of a heating chamber, a turntable motor for rotating a turntable, and a fan motor for cooling a magnetron 6—that need to be operated as high-frequency heating is performed is connected to the node between the time-limiting switches 1e and 1f. Moreover, the other end of the high-frequency heating oscillator cooling device and other components 3 is connected to the node between the other end of the commercial alternating-current power source 2 and to the other end of the primary coil of the high-voltage transformer 5.
Next, the components connected to the secondary side of the high-voltage transformer will be described. Between the anode and cathode of the magnetron 6 is connected a diode D1 in parallel therewith. Specifically, to the anode of the magnetron 6 is connected the cathode of the diode D1, and to the cathode of the magnetron 6 is connected the anode of the diode D1. Moreover, to the cathode of the magnetron 6 is connected a secondary coil 5a of the high-voltage transformer 5. Furthermore, to the anode of the diode D1 is connected, via a capacitor C1, one end of a secondary coil 5b of the high-voltage transformer 5, and to the cathode of the diode D1 is connected the other end of the secondary coil 5b. The anode of the magnetron 6 is grounded.
Now, the operation of the microwave oven configured as described above will be described. The heating duration setter 1a has a rotary knob (not illustrated). When the user rotates the rotary knob clockwise, the heating duration setter 1a sets a heating duration commensurate with the amount of rotation. As the heating duration passes by, the rotary knob rotates counter-clockwise by a rotation angle commensurate with the lapsed time, thereby indicating the remaining heating duration on an analog basis. The time-limiting switch 1e remains on during the heating duration, and otherwise remains off. On the other hand, the time-limiting switch 1f, during the heating duration, toggles between on and off with a duty factor determined by the motor and gear-and-cam mechanism (not illustrated) incorporated in the heating duration setter 1a, and otherwise remains off. At the end of the heating duration, the bell 1b sounds.
When the time-limiting switch 1e is on, i.e. during the heating duration, electric power is supplied from the commercial alternating-current power source 2 to the high-frequency heating oscillator cooling device and other components 3, so that the high-frequency heating oscillator cooling device and other components 3 operate.
When the time-limiting switch 1e is on and the time-limiting switch 1f is on, electric power is supplied from the commercial alternating-current power source 2 to the high-voltage transformer 5, so that a high voltage of about 4 kV appears at the secondary side of the high-voltage transformer 5. This high voltage is supplied to the magnetron 6, so that the magnetron 6 oscillates a microwave. A target to be heated is irradiated with this microwave, and is thereby heated. Here, through the time-limiting switches 1e and 1f flows a current that is needed to achieve microwave heating, and therefore the time-limiting switches 1e and 1f need to be mechanical switches through which a current of at least 15 A can be passed. On the other hand, when the time-limiting switch 1e is on and the time-limiting switch 1f is off, no electric power is supplied from the commercial alternating-current power source 2 to the high-voltage transformer 5, so that the magnetron 6 oscillates no microwave. Thus, the microwave output is determined by the duty factor mentioned above.
If the timing with which the time-limiting switch 1f turns from off to on is not in synchronism with the phase of the commercial alternating-current power source 2, the exciting current of the high-voltage transformer 5 may produce a large rush current, over 100 A in the worst case. It is for this reason that, in the conventional microwave oven shown in FIG. 6, which cannot bring the timing with which the time-limiting switch 1f turns from off to on into synchronism with the phase of the commercial alternating-current power source 2, the surge circuit 10 is provided with a view to suppressing rush current.
The switch 12 is controlled by the surge input monitoring circuit 11 so as to be normally on, short-circuiting the resistor R2. The surge input monitoring circuit 11 monitors the value of the rush current, and, when the monitored rush current becomes higher than a threshold value, keeps the switch 12 off for a predetermined period. When the switch 12 is off, the rush current is branched via the resistor R2, reducing the effect thereof.
It is true that providing the surge circuit 10 as described above helps reduce rush current. However, even the surge circuit 10 cannot minimize rush current, and thus a heavy burden remains imposed on the time-limiting switch 1f, through which a large rush current flows as usual when it turns from off to on.
Moreover, the surge circuit 10 is composed of rather large components. Thus, even in a microwave oven provided with a circuit board on which to mount electric components, unlike the other components mounted thereon, the surge circuit 10 is not mounted on the circuit board, but is fitted to the main unit of the microwave oven. This necessitates an extra step of fitting the surge circuit in the manufacturing procedure, and thus hinders cost reduction. Moreover, the large components of the surge circuit 10 hinders size reduction.
On the other hand, there have conventionally been known also high-frequency heating apparatuses, for example the one disclosed in Japanese Patent Application Laid-Open No. S63-205088, in which a switch for controlling the supply of electric power to a high-voltage transformer is controlled by a microcomputer so as to reduce rush current. However, this type of high-frequency heating apparatus is not provided with a mechanical time-limiting device that permits the user to visually recognize the remaining heating duration in the form of the amount of rotation of a rotary knob. Thus, to permit the user to visually recognize the remaining heating duration, it is necessary to additionally provide a display device such as a liquid crystal display. Additionally providing such a display device leads to higher cost.